This invention relates to a novel powder having the quality of intumescence at relatively high temperature, the powder having a useful range of formulations, and to the method for its production.
Intumescent powders when heated above their temperature of intumescence, generate a multitude of glassy bubbles and swell to many times their original volume. The intumescent powders are used where such swelling has a useful effect in preventing the spread of fire, closing passages through floors and walls in a fire environment, and other uses.
By xe2x80x98plasticxe2x80x99 herein I refer to the state of a thermoplastic when it may be shaped, molded or formed.
Intumescent powders used in the past were commonly sodium silicate: xSiO2:Na2O where xe2x80x98xxe2x80x99 represents the fact that the proportion of SiO2 to Na2O in such powder may vary. The sodium silicates used began to intumesce at 120xc2x0 C.-140xc2x0 C. Thus any binder or matrix material heated to be plastic for mixing with the sodium silicate powder must have had a plastic point below the intumescence point of the silicate to avoid premature intumescence. For thermoplastic materials, this limited the compounds for mixture with the intumescent powders to such materials as: rubber, latex, silicone being materials plastic at or below 120xc2x0 C.-140xc2x0 C.
By controlling the xSiO2, a Li2O: d(Na2O+K2O) molar ratio as later defined herein and controlling moisture content to about 8%-12% by weight and further to grind the dried powder to between about 50 to 500 microns, we have developed a powder with a high degree of expansion (up to 110 times original size). Intumescence in the novel powder does not occur until about 195xc2x0 C.-220xc2x0 C. which is well above the compounding temperature of many thermoplastics at about 170xc2x0 C.-185xc2x0 C. For use in the inventive powder, sodium and potassium are almost equivalent on a mole to mole basis so it is not thought necessary to specify them separately, but merely to specify the content by molecular fraction of the sum of the sodium and potassium molar values relative to the other ingredients. (Although most applications require that the powder be ground to 50 to 500 microns we have used the powder ground to as fine as 20 microns for coating fine wire in a suitable vehicle.)
Thus the intumescent powder can be compounded with many thermoplastic materials (e.g. polyethylene, polyvinyl chloride (PVC), and more with plastic temperatures at up to 170xc2x0 C.-185xc2x0 C. This has not been achieved before because the silicates formerly used: (sodium silicate and potassium silicate) both begin to intumesce at about 120xc2x0 C.-140xc2x0 C.
The ability of sodium and potassium silicates to intumesce at temperatures around 120xc2x0 C. is well known. Sodium and potassium silicates are water soluble glasses. When they contain 8-15% water, by weight, these glasses begin to soften at about 100xc2x0 C.-150xc2x0 C. At about the same time the attached water begins to vaporize. The compound effect is called intumescence which results in the formation of numerous small glass bubbles, or a glass foam.
Lithium silicate, however, intumesces very little. The reason is that hydrated lithium silicate has a much higher softening point (about 230xc2x0 C.) than do hydrated potassium and sodium silicates. By the time the hydrated lithium silicate softens, much of the water has already vaporized, thus little or no intumescence is possible. (In this new development sodium silicate behaves like potassium silicate mole for mole. Thus the reference is to xe2x80x9csodium and/or potassium silicatexe2x80x9d referring to the fact that or that any combination of the two may be used in the correct molar proportion.)
By combining the sodium and/or potassium silicates, on the one hand and lithium silicates on the other, in the molar ratios indicated, we created a hydrated sodium and/or potassium lithium silicate with a softening part of about 195xc2x0 C.-220xc2x0 C. This is approximately the temperature at which the water ionically bound to the lithium, is released as water vapor. Because the release of water concurs with the softening point of the sodium and/or potassium with lithium silicate, the resulting intumescence is maximized (an increase in size of up to 80-110 times by volume. (The water originally ionically bound to the sodium, and/or potassium was released beginning at about 120xc2x0 C. at which time the Naxe2x80x94Li silicate had not yet softened. Consequently little or no intumescence occurs below 195xc2x0 C.)
This invention provides a powder which intumesces at a temperature of about 195xc2x0 C.-220xc2x0 C. Thus the powder may be mixed without intumescence with thermoplastic material having a plastic temperature lower than 195xc2x0 C. to make a moldable material having the ability to intumesce at temperatures of about 195xc2x0 C.-220xc2x0 C. suitable for wide varieties of uses. Suitable thermoplastic materials for this purpose include polyvinyl chloride (PVC) which softens near 175xc2x0 C., and polyethylene which melts at 165xc2x0 C. Other thermoplastics with plastic points below 195xc2x0 C. include ethyl vinyl acetate (EVA), acrylonitrile, butadiene styrene (xe2x80x98ABSxe2x80x99) copolymers, polypropylene, acrylic polyethylene, ethyl vinyl alcohol, thermoplastic polyester.
The novel intumescent powder may be mixed with other compounds if desired if this mixing can be done at temperatures less than the intumescence temperature of the powder. The powder may be applied as a coating to metal in a suitable vehicle such as urethane. The intumescent coating on the metal intumesces if the coating is exposed to 195xc2x0 C.-220xc2x0 C., and the intumesced powder provides a protective coating for the metal. In a different use, the intumescent powder is combined with a matrix of material. Upon exposure to temperatures above 195xc2x0 C.-220xc2x0 C. the powder intumesces and creates a foamed effect in the matrix. This foamed matrix may act as an emergency float in water and hence may be attached to panels or other equipment to create flotation of the equipment in the event the equipment falls into the water.
There is described herein a mixed Li, and Na and/or K alkali silicate powder containing significant lithium oxide, with a high degree of intumescence and a temperature of intumescence above the plastic state temperature of thermoplastic materials, with which the powder is to be mixed, such as those listed above in paragraph [0010].
The alkali silicate powder is a mixed Li/Na and/or K silicate. (For use in the inventive powder sodium and potassium cations have similar behaviour mole by mole as far as the inventive forms as the formulation characteristic is concerned, so that it is not necessary in connection with this invention to specify them separately but merely to specify the content by total molecular weight of the sodium and potassium molar values relative to the content of the other ingredients.
Two main factors influence the characteristics of the mixed Li:Na:K silicates of the invention. One is the silica to cation oxide molar ratio which can be expressed as the ratio y SiO2:M2O where y is part of the numerator of the ratio when the denominator is 1; and where M2O (equal to 1) is the total metal oxide which may include a Li2O, b Na2O, and c K2O when a, b, c are the relative molar quantities of the cation oxides and total 1.00.
The other is the ratio of cation oxide a Li2O: b Na2O: c K2O. The molar fractions of a Li2O, b Na2O, and c K2O total units as the denominator (M2O=1) for the ratio SiO2:M2O in the ratio described.
In view of the similarity in effects between sodium and potassium, the molar ratioxe2x80x94a Li2O, b Na2O, and c K2O may be written as: a Li2O: d(Na2O+K2O)(where d is the sum of b and c in moles) and a+d=1.00.
For brevity it is sometimes herein desired to indicate the mixture numerically only. For example a ratio 3.00/0.31:0.33:0.35 shall be read herein to indicate 3.00 SiO2/0.31 Li2O:0.33 Na2O:0.35 K2O and the ratio 3.00/0.31:0.69 shall be interpreted to indicate the molar ratio 3.00 SiO2/0.31 Li2O:0.69 (Na2O+K2O).
The tests conducted on the water solubility of ternary Li/Na/K silicates indicate that potassium and sodium silicates are similar in solubility.
The formulations of the intumescent powder within the scope of the invention range over the molar ratios (2.20 to 3.70)SiO2/(0.20-0.35)Li2O:(0.80 to 0.65)(Na2O+K2O) where the coefficients of Li2O and (Na2O+K2O) are chosen so that the sum of the coefficients equals 1.00 and where the molar ratios are accurate to plus or minus 2%. The powder is dried to a moisture content of 8-12% and usually ground to 50 to 500 microns.
The formulation ratios are accurate to about 2 parts in 100 which is the commercial standard of accuracy for the materials specified, which constitutes the allowable variation in the molar ratios of the specification and claims.
The preferred limits of the formulations may be commented on as follows. For values of SiO2 below 2.20 the temperature of intumescence is lower than desired. Also the product tends to be less water resistant, and more readily efflorescent and more hygroscopic. For molar values of SiO2 above 3.70 the degree of intumescence is less than desired although there is good water resistance, low efflorescence and less hygroscopic effect.
For values of Li2O less than 0.20, the main proportion of water remaining after drying (discussed hereafter) is linked to the Li2O, the decrease in the Li2O below 0.20 contemplated above has a consequent reduction of water so that the remaining water may be insufficient to cause a desirable degree of intumescence.
For values of Li2O greater than 0.35 the degree of intumescence is reduced below the limits desired for combining with thermoplastics.